Receiving data in a sensor network

ABSTRACT

A sensor network has a plurality of wireless sensors which transmit to an intermediate receiving device which relays data to a central server. A method is provided for receiving data packets at the intermediate receiving device from a plurality of the transmitting devices. Data packets are sensed on a communication medium at the receiving device and the total traffic intensity of data packets from the transmitting devices is estimated. A detection threshold for data packets is provided and adapted as a function of the total intensity. The receiving device receives data packets with a signal strength above the current detection threshold.

TECHNICAL FIELD

This invention relates to the field of receiving data in a sensornetwork and it particularly relates to cluster head design intransmit-only wireless sensor networks.

BACKGROUND OF THE INVENTION

There has been a recent increase in demand for wireless sensor networks,due to their simplicity, low cost and easy deployment. Wireless sensornetworks can serve different purposes, from measurement and detection,to automation and process control.

A typical wireless sensor network consists of sensor nodes (SNs) andsinks. SNs are wireless nodes equipped with sensing devices whose goalis to gather data in the environment and transmit it to a centralserver. It is advantageous if the transmission is wireless, since thenumber of sensors is typically very large, and the cost of deployment ofa wired infrastructure would be more expensive.

In order to have a longer lifetime, SNs typically use smallertransmission powers. The area covered by a sensor network may be largeenough that intermediate devices are useful to relay data. These devicesare data sinks and are called cluster heads (CHs). A CH is a devicewhose task is to capture transmissions of SNs in its environment,optionally do some limited processing of the data, and to forward thedata to a central server. One CH is responsible for coordinating anumber of SNs: for a network of 100 SNs. there may be less than 10 CHs,depending on the network area.

Since there are much fewer CHs than SNs, they can be more expensive.They can rely on more sophisticated wireless technology to transmit datato the central server, or in some cases they can also be wired.

In many applications of sensor networks, a network is desired to copewith bursts of high data rates. One example is an intrusion detectionnetwork, where once an intruder is detected, a number of sensors,including video cameras, are activated to inform about the intrusion. Asimilar example is a fire detection problem. In these applications it isseen that, even though the average packet rates are relatively low e.g.since intrusion or fire events are typically rare events, a network isdesired to be able to support higher rate bursts.

SUMMARY OF THE INVENTION

The present invention provides a sensor network for lower averagedata-rates, which is able to better handle sudden bursts of traffic.According to a first aspect of the present invention there is provided amethod for receiving data packets from a plurality of transmittingdevices at a receiving device, comprising: sensing data packetstransmitted on a communication medium; estimating the total trafficintensity of data packets generated by the transmitting devices; settinga detection threshold for receiving data packets as a function of thetotal traffic intensity; ignoring data packets with a received signalstrength below the detection threshold; and receiving data packets witha received signal strength above the detection threshold.

According to a second aspect of the present invention there is provideda receiving device for receiving data packets from a plurality oftransmitting devices, comprising: a receiving circuit for receiving datapackets with a received signal strength above a current detectionthreshold; estimator for the total traffic intensity of data packetsgenerated by transmitting devices; controller for setting the detectionthreshold for receiving data packets as a function of the total trafficintensity.

According to a third aspect of the present invention there is provided acomputer program product stored on a computer readable storage medium,comprising computer readable program code means for performing the stepsof: sensing data packets transmitted on a communication medium;estimating the total traffic intensity of data packets generated by thetransmitting devices; adapting a detection threshold for receiving datapackets as a function of the total intensity; ignoring data packets witha received signal strength below the current detection threshold; andreceiving data packets with a received signal strength above the currentdetection threshold.

According to a fourth aspect of the present invention, there is provideda sensor network comprising: a plurality of wireless sensors havingtransmitters; one or more receiving devices; and a central server forreceiving data from the one or more receiving devices; wherein the oneor more receiving devices comprising: a receiving circuit for receivingdata packets from the sensors, the data packets having a signal strengthabove a current detection threshold; an estimator for the total trafficintensity of data packets generated by the sensors; a controller foradapting the detection threshold for receiving data packets as afunction of the total traffic intensity; and a transmitter fortransmitting received data to the central server.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its embodiments will be more fully appreciated byreference to the following detailed description of presentlyadvantageous but nonetheless illustrative embodiments in accordance withthe present invention when taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic diagram of a sensor network;

FIG. 2 is a block diagram a cluster head and a sensor node;

FIG. 3 is a schematic diagram illustrating a detection threshold;

FIG. 4 is a graph showing aggregate input traffic against the aggregateoutput of a system;

FIG. 5 is a diagram illustrating data packet arrivals at a cluster head;

FIG. 6 is a flow diagram of a method of receiving data at a clusterhead;

FIGS. 7A and 7B are flow diagrams of further details of a method ofreceiving data at a cluster head;

FIG. 8 is a block diagram of a cluster head and a sensor node with twodetection and synchronization circuits; and

FIG. 9 is a flow diagram of a method of receiving data at a cluster headdesigned according to FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides methods, apparatus and systems for asensor network for lower average data-rates, that are able to betterhandle sudden bursts of traffic. A goal of improved cluster head designis to increase the number of captured packets. There are two differenttraffic cases, one being referred to as low network traffic, the otheras high network traffic. The distinction between these two cases can bemade by defining an intensity threshold. For example low network trafficcould be defined as the case when portion of time that a cluster headdoes not sense a packet transmission is larger than the time duringwhich it does sense a packet transmission. In contrast thereto then highnetwork traffic might be defined as the case when the portion of timethat a cluster head does not sense a packet transmission is shorter thanthe time during which it does sense a packet transmission. There can beused other definitions, for instance those that define the intensitythreshold in dependence on an overlap probability between two or morepackets being sensed at a cluster head.

In the case of low network traffic, each cluster head should try tocapture each transmitted packet since packets transmissions are ratherrare. Should one cluster head drop the packet due to fading orinterference, there is a chance that other cluster heads will captureit. However, when the traffic is high, the reasoning is different. If acluster head tries to decode a packet from a distant sensor, there is ahigher probability that another interfering packet will arrive in themeantime from a closer sensor. The interfering packet will destroy thecontent of the receiving packet due to its overwhelming power.Furthermore, since the cluster head was busy receiving the packet fromthe distant sensor, it did not even try to receive the interferingpacket and it ends up loosing both packets.

The present invention is concerned with cluster head design. Astraightforward solution to the above problem is to have multiplereceiver circuits at each cluster head. However, receiver circuits areexpensive. The invention proposes a solution that mitigates the problemand does not drastically increase the cost of a cluster head.

A first architecture of the present invention is based on a notion of adetection threshold. If the received signal strength of a packet at acluster head is below a detection threshold, the cluster head will noteven try to receive the packet, and thus will not keep the circuitsconnected to it busy during the duration of the packet. An algorithm isproposed that tracks and adapts the detection threshold at a clusterhead, as a function of total traffic, in order to increase the captureprobability. When the traffic is low, the detection threshold is low,and in the ideal case all the packets are detected. When the trafficincreases, the threshold increases gradually to keep the performance ata predetermined level. This predetermined level will in an ideal case bea level trying to capture as many packets in an uncorrupted state aspossible. Although the hardware requirements are the same as for thesimple receiver, this approach can bring significant improvements duringburst periods. In the context of the invention, low traffic is referredto as traffic whose traffic intensity lies below a predeterminedintensity threshold, while high traffic is traffic whose intensity liesabove that intensity threshold. A possible determination rule for theintensity threshold could be to set the threshold at a value at whichwithin a predetermined time window the aggregated time during which thecluster head does not sense any packet, also referred to as idle time,is above or below e.g. 20% of the time window duration. More datatraffic means a shorter idle time; hence the traffic falls within thehigh data traffic category. Less data traffic means a longer idle time,hence the traffic falls within the low data traffic category. Any otherdetermination rule can be used as well to determine the trafficintensity threshold.

A second architecture of the present invention is referred to asswitched architecture. An additional detection and synchronizationcircuit is added to the existing receiver's design. The goal of thisadditional circuit is to detect an arrival of a packet whose signalstrength is stronger than the one currently being decoded. Should ithappen, the main receiver circuit drops the current packet and switchesto the stronger one. Obviously, the hardware cost of this receiver ishigher than the cost of the simple one. However, this approach brings anadditional performance improvement over a cluster head with adaptivethreshold. It is thus suited for scenarios where the improvements ofcluster heads with adaptive thresholds are not sufficient.

Thus, the present invention provides a method for receiving data packetsfrom a plurality of transmitting devices at a receiving device,comprising: sensing data packets transmitted on a communication medium;estimating the total traffic intensity of data packets generated by thetransmitting devices; setting a detection threshold for receiving datapackets as a function of the total traffic intensity; ignoring datapackets with a received signal strength below the detection threshold;and receiving data packets with a received signal strength above thedetection threshold.

Preferably, when the total traffic intensity is lower than apredetermined intensity threshold, the detection threshold is decreased.By this decreasing of the detection threshold, typically more packetswill be sensed such that the traffic intensity will increase. When thetraffic intensity then increases over the intensity threshold, thedetection threshold is increased again. Thereby the traffic intensitycan be held within a predetermined range around the intensity threshold.Thereby the probability of packet loss can be held at a predeterminedlevel even with varying traffic intensity. The detection threshold maybe adapted for every data packet sensed. So the method is suited to keepthe traffic intensity at a predetermined level.

The method may include tracking and estimating the traffic intensity andthe utilization at the receiving device and adapting the detectionthreshold to keep the utilization at a predetermined level. The trafficintensity may be estimated by a packet duration divided by the averageidle time at the receiving device. The utilization at the receivingdevice may be estimated by a packet duration divided by the average timebetween two successful packet receptions. The estimated trafficintensity and utilization may be continually updated, i.e. updated atregular or irregular time intervals, e.g. after a predetermined numberof packets received. The method may include maintaining a list of activetransmitting devices, i.e. devices whose packets are sensed and theirreceived signal strengths.

For increased performance the total traffic intensity is preferablymaintained at approximately 75% of the physical layer rate and theutilization is preferably maintained at approximately 22%.

The detection threshold may be conservatively increased by onlyincreasing the threshold if the traffic intensity has been detected tobe increasing over a number of instances.

The detection threshold may be conservatively increased in that thedetection threshold is increased if it is detected a predeterminednumber of times in succession that the total traffic intensity isgreater than 75% and the utilization is less than 22%. The detectionthreshold may be decreased if the total traffic intensity is less than75% and the utilization is less than 22%.

The present invention also provides a receiving device for receivingdata packets from a plurality of transmitting devices, comprising: areceiving circuit for receiving data packets with a received signalstrength above a current detection threshold; estimator for the totaltraffic intensity of data packets generated by transmitting devices;controller for setting the detection threshold for receiving datapackets as a function of the total traffic intensity.

The device may include a sensor for data packets on a communicationmedium at the receiving device. The device may also include a list ofactive transmitting devices and their received signal strengths.

The present invention further provides a computer program product storedon a computer readable storage medium, comprising computer readableprogram code means for performing the steps of: sensing data packetstransmitted on a communication medium; estimating the total trafficintensity of data packets generated by the transmitting devices;adapting a detection threshold for receiving data packets as a functionof the total intensity; ignoring data packets with a received signalstrength below the current detection threshold; and receiving datapackets with a received signal strength above the current detectionthreshold.

The present invention still further provides a sensor networkcomprising: a plurality of wireless sensors having transmitters; one ormore receiving devices; and a central server for receiving data from theone or more receiving devices; wherein the one or more receiving devicescomprising: a receiving circuit for receiving data packets from thesensors, the data packets having a signal strength above a currentdetection threshold; an estimator for the total traffic intensity ofdata packets generated by the sensors; a controller for adapting thedetection threshold for receiving data packets as a function of thetotal traffic intensity; and a transmitter for transmitting receiveddata to the central server. The network may operate on a pulse-basedultra-wide band physical layer.

Referring first to FIG. 1, a described embodiment of a wireless sensornetwork 100 is shown. The sensor network 100 comprises multiple sensornodes (SNs) 102 which are wireless nodes equipped with sensing devicesand transmitters. The sensor network 100 includes intermediate devicesin the form of cluster heads (CHs) 104. Each CH 104 receives data fromSNs 102 in the local vicinity of the CH 104 and relays the data to acentral server 106. The CHs 104 may optionally carry out some processingof the data captured in the transmissions from SNs 102 before relayingthe data to the central server 106. The CHs 104 may use wirelesstechnology for communication with the central server 106, or may usewired links.

The physical layer of the network of the described embodiment of asensor network 100 is based on non-coherent pulsed UWB (ultra-wide band)radio. The advantages of this radio are that it has a lower designcomplexity, provides sufficiently high data rates (for example, 5 Mbpslink rate), and has a lower power dissipation and better rangingcapabilities. It comprises sending pulses of the order of 1 ns duration.Sensor networks 100 based on pulse-based UWB are location-aware, whichis a desirable feature for applications such as location tracking andintrusion detection.

Furthermore, implementing a transmitter is simpler and cheaper, evencomparing to a receiver for the same radio. Since it is desirable toprovide low-cost networks, in order to further decrease the cost, areceiver is not implemented in a SN 102, but only a transmitter. Thismakes the SNs 102 unaware of the state of the network 100, and thenetworking logic is implemented in the CHs 104. However, in alternativeembodiments. the SNs 102 of a network 100 may also include datareceivers.

It is noted that the described sensor network uses a non-coherent pulsedUWB radio physical layer. However, the described design may be used withany networks with transmit-only SNs.

The sensor network 100 comprises multiple SNs 102, which can amount to arelatively large number. These SNs 102 are therefore desired to bedesigned rather simply and at a reduced cost. A sensor network 100should ideally be able to cope with relatively high data rates and offerlocation capabilities, and focus the SNs 102 with pulse-based UWBphysical layer. Pulse-based UWB transmitters are cheap and simple toimplement; nevertheless, even a simple, non-coherent receiver, usescomplex elements, such as a synchronization circuit, and may be tooexpensive for low-end SNs.

In the described embodiment a network of transmit-only SNs 102 isprovided, equipped with sensing and transmitting devices. These SNs 102measure some data and transmit it to CHs 104. They can use an arbitrarymedium access scheme, but this scheme depends only on the availabledata. The SNs 102 cannot sense the medium nor can they receive anyfeedback from CHs 104 or other SNs 102.

The choice of sensor architecture implies that most of the designcomplexity is in the CHs 104.

Sensor networks 100 are usually low-data-rate networks. The main reasonis that low traffic, hence low average data rates imply lower powerdissipation and longer network lifetime. However, even with a lowaverage data rate, the peak traffic may still be high, but only duringinfrequent time intervals. A network 100 should be designed in such away that it can increase its performance both during low trafficintervals and high traffic bursts.

An example embodiment of a sensor network 100 is a surveillance system.An underground car park is filled with sensors. There are several typesof sensors: seismic, infrared and microphone sensors are used to detectmovements of an intruder. In addition, there are video sensors that areactivated only if some activity has already been detected. SNs 102transmit data to CH 104, that collect it and forward it to a centralserver 106. Typical networks 100 of this type consist of 10-100 SNs,with rates up to 400 kbps when video transmissions are activated. Theaggregated input traffic thus goes up to 40 Mbps for the whole network.In FIG. 1, intrusion movement 110 is depicted with a thick black lineand the shaded surface 112 denotes the region in which SNs 102 areactivated by the intruder.

In another example embodiment, the network 100 is a position monitoringsystem for training purposes. A typical network contains 100 SNs andmaximal rate is 100 kbps. A further example embodiment is a shelfmanagement and monitoring system. The system is designed to accommodateup to 1000 SNs with rates of 10-100 kbps. Although all nodes will hardlyever all be active at the same time, the total aggregated input trafficof a network can easily go to tens of Mbps.

Another example is a fire detection sensor network. Sensors aredistributed on an area, and their goal is to detect a fire and tomonitor its spreading. The average traffic in such a network is of thelow traffic type. However, in case of fire, there is a burst of trafficfrom those SNs that detect fire. Most of the information is redundant tosome extent: it is sufficient to get packets from one sensor to detectfire. In order to get more precise information on fire spreading, moredata packets should be captured.

Summarizing the above facts, sensor networks 100 may have lower averagedata rates and a multiplicity of nodes, but may be confronted withhigher peak data rates. A network architecture is proposed to betterhandle the burst periods of peak transmission rates, defined by theseexamples, and which will also be efficient in low-traffic periods.

SNs 102 are designed to use given coding and modulation schemes. Theseschemes are fixed since SNs 102 do not interact with the environment. Areception of a packet at a CH 104 will be successful if the signalstrength is high enough, and if the level of the interference comingfrom concurrent transmissions is low enough.

The goal of the CHs 104 is to successfully receive packets from SNs 102.If a sensor network 100 is lightly loaded, the preferred strategy of aCH 104 is to try to decode every packet it senses on the medium. SinceCHs 104 do not control sensors' medium access, they cannot preventtransmission failures that occur due to collisions.

However, the story is different when a network load is higher. A typicalwireless receiver has only a single receiving circuit, thus can receiveonly one packet at a time. While a CH 104 is receiving a packet from adistant SN 102, another transmission may start from a near-by SN 102.This transmission will interfere and may corrupt the already receivedpacket. At the same time, the CH 104 will not be able to receive theinterfering packet since its receiving circuit was busy when thetransmission started. Hence, both packets will be corrupted and lost.

This problem can be overcome if a CH 104 is equipped with severalreceiving circuits. However, such a solution is rather expensive andmore difficult to implement.

Two CH architectures are described. The first CH architecture is basedon an adaptive detection threshold. Each CH 104 has a single receivercircuit with a detection threshold which is adapted as a function of thetraffic sent by the SNs 102. It starts receiving a packet only if thereceived signal strength is above the detection threshold. In addition,the CH 104 tracks the SNs' 102 traffic intensity and constantly adaptsthe detection threshold.

The second CH architecture assumes that each CH 104 contains onereceiving circuit, and an additional detection and synchronizationcircuit. A CH 104 starts receiving the first packet it observes on thewireless medium. The goal of the additional detection circuit is tomonitor the medium in parallel, and to detect if a packet, stronger thanthe one currently being received, appears. If this happens, thereceiving circuit drops the ongoing transmission and switches to thestronger packet.

Referring to FIG. 2, a first CH architecture is shown. A CH 104 has asingle receiving circuit 202 and a variable detection thresholdcontroller 204. The variable detection threshold controller 204 detectsthe level of traffic from SNs 102 and determines a variable detectionthreshold, which is herein denoted with P_(dt), which varies with thelevel of traffic. If the signal strength of a received packet from a SN102 is lower than P_(dt), then the packet will be ignored. Otherwise,the CH 104 will try to receive it. A packet is “detected” if thereceived signal is stronger than P_(dt), only then will the CH 104 tryto receive it.

For simplicity of presentation, it can be assumed that the signalattenuation is a time-invariant function of distance and that all SNssend with the same power. Referring to FIG. 3, a CH 104 is shown withsurrounding SNs 102. There exists a threshold region of radius R_(dt),such that if a SN 102 is outside this region, a CH 104 will not startreceiving packets sent by this SN 102. In FIG. 3, transmissions of SNs102 whose received signal at the CH 104 is higher than P_(dt) aredenoted with solid lines 302. The transmissions of SNs 102 whosereceived signal is below P_(dt) are denoted with dashed lines 304. Theequivalent detection region of radius R_(dt) is represented with ashaded circle 306.

The main reason why a CH does not want to start receiving a packet isthat if it starts receiving a packet whose signal strength is relativelylow, there is a higher probability that the packet will be dropped dueto collision. In the meantime, other packets, possibly with highersignal strength, will be rejected since the only receiving circuit isbusy.

A drawback of this approach is that distant SNs, i.e. those outside thecircle 306 will not be able to convey any information at all to thecentral server. However, this drawback is a consequence of theconstraints on the SN architecture, and not of the protocol. Since SNscannot adapt their medium access policy, there is no way to receivepackets from distant SNs when the traffic is high, regardless of the CHarchitecture. Nevertheless, this problem can be mitigated in severalways. Firstly, a burst of traffic is usually triggered by an event.Therefore, even if some packets from distant SNs are dropped, not toomuch information might be lost due to a correlation among data. However,if a reliability of information is crucial, a simple solution is to addmore CHs on critical places, to increase the capture probability. Sincethe number of CHs is anyway expected to be lower than the number of SNs,the solution will be cheaper than implementing a receiver in each SN.

In order to better understand this issue, a simple model of the systemhas been defined and analyzed analytically. It is assumed that thetraffic distribution of every SN is Poisson. This is a reasonableassumption, since if a system is heavily loaded; the preferred mediumaccess for every SN is to defer each transmission for some random time.A simplified physical layer model is also assumed: if a CH receives apacket from a node with signal strength P^(rcv), and if an interferingpacket, which overlaps even for a small fraction of time, comes withsignal strength larger than P^(rcv)−Δ, then the received packet will belost. This approximation has been tested on a physical model usingnetworks with 2 nodes, and it was found that the approximation holds.This simplification neglects the impacts of multiple concurrentinterferers, but it fits well with the simulated results.

The scenario depicted in FIG. 3 is considered with one CH and n SNs.Packets from SN i are received at the CH with signal strength P^(rcv)_(i). Each SN i generates a Poisson traffic with distribution λ_(i). Letthe detection threshold be P_(dt), which means that it is tried todecode packets whose received signal strength is larger than P_(dt). Thetotal traffic generated by the nodes is:${\lambda_{a}\left( P_{dt} \right)} = {\sum\limits_{i:{{P^{rcv}i} \geq P_{dt}}}\lambda_{i}}$

The probability that the receiver is idle at any given moment in time isestimated. The receiver is idle if it has finished decoding a previouspacket (successfully or unsuccessfully), and if no other packet hasarrived in the meantime with received signal strength larger thanP_(dt). The state of receiver (busy or idle) is a stationary process intime the probability of receiver being idle is called P^(rcv) _(idle).This process can be described with a continuous Markov chain giving thestationary probability P^(rcv) _(idle)=1/(1+λ_(a)(P_(dt)))

Next, the probability that a SN i will successfully transmit a packet ismodeled. This will happen if the receiver is idle at the time a packettransmission starts, and if the packet does not overlap with any packetwhose received signal strength is higher than P^(rcv) _(i)-Δ. It isassumed all packets have a fixed length and that the packet transmissiontime is one. Similar to non-slotted Aloha, if a packet arrives at time0, then any other packet arriving within [−1, 1] will interfere with itand cause collision. Therefore, the packet capture probability of node iis:${P_{capture}\left( {i,P_{dt}} \right)} = {P^{rcv}{{idle}\left( P_{dt} \right)}{\exp\left( {{- 2}\sum\limits_{j:{{P^{rcv}j} \geq {{P^{rcv}i} - \Delta}}}} \right)}}$

The average throughput of SN i is then λ_(i)P_(capture)(i, P_(dt)) andthe average throughput of all SNs is: $\begin{matrix}{\quad{{\overset{\_}{X}\left( P_{dt} \right)} = {\sum\limits_{i:{{P^{rcv}i} \geq P_{dt}}}{\lambda_{i}{P_{capture}\left( {i,P_{dt}} \right)}}}}} & (1)\end{matrix}$

The problem of maximizing equation (1) can be solved numerically. For amultitude of topologies and traffic distributions, it has been foundthat it is always optimal to maintain a total traffic load λ_(a)=0.75which yields the efficiency of the medium {overscore (X)} of 22%. Inother words, λ_(a)(P_(dt)) should preferably be estimated and P_(dt)varied to obtain λ_(a)=0.75.

As described above, it is considered preferable to keep λ_(a)=0.75. Amethod is described to track the load λ_(a) and utilization of thesystem {overscore (X)}, and to adapt P_(dt) in order to keep theutilization at an increased level, ideally at the maximum level.

The model is verified by simulations. 50 SNs are randomly distributed ona 40 m×40 m square with one CH in the middle. FIG. 4 shows a graph 400with the aggregate input SN traffic on the x axis 401 and the aggregateeffective output rate on the y axis 402 The physical link rate is 5Mb/s. It can be seen in FIG. 4 that the effective output rate is maximalwhen the aggregate traffic is around 75% of the physical link rate. Atthat point, the effective output rate of the system is around 22% of thephysical link rate.

A method is proposed to track load λ_(a) and utilization of the system,and to adapt P_(dt) in order to keep the utilization at a predeterminedlevel.

FIG. 5 shows packets arriving at a CH shown along a timeline 400 andillustrates the adaptation mechanism. Packet 1 501 has arrived and iswell received. Packet 2 502 was being received when it collided withpacket 3 503 and is discarded. Packet 4 504 is not detected because itarrives from a node outside a detection threshold. Finally, packet 5 505again is well received.

First, λ_(a), is estimated. As shown in FIG. 5, T_(l) is denoted as theaverage idle time at receiver that is the average time between twopackets from sensors that are detected. The end time T_(end) of the lastdetected packet (successfully or unsuccessfully received) is monitoredand at the moment T_(detect) when a new packet is detected, an update ofthe idle time is performed: T_(l)=αT_(l)+(1−α)(T_(detect)−T_(end)). Theestimate of the intensity of detected load is then λ_(a)=packetduration/T_(l).

A similar thing is done for estimating utilization {overscore (X)}.T_(u) denotes the average time between two successful receptions. Thetime of the end of the last successful packet transmission is keptT_(end) _(—) _(succ). At the instant of the next successful packettransmission T_(next) _(—) _(succ), T_(u) is updated. For updating,exponential weighted average is used T_(u)=αT_(u)+(1−α)(T_(next) _(—)_(succ)−T_(end) _(—) _(succ)). The estimate of the utilization is then{overscore (X)}=packet duration/T_(u). The filter constant is set α=0.95in both cases.

In parallel, a CH also learns about existing SNs and their distances.This is done during packet receptions. A CH does not need necessarily toreceive successfully a whole packet to perform this estimate. It mightbe sufficient to decode the header, and to estimate signal strength. Asa result of this estimation a CH keeps a list of active sensors andtheir received signal strengths {i, P^(rcv) _(i)}_(i=1, . . . ,n),ordered decreasingly by signal strengths.

A preferable setting of the algorithm is to keep utilization at 22% anddetected load at 75%. In theory it should be sufficient to use detectedload as an estimator, but using both detected load and utilizationprovides better robustness. The adaptive threshold algorithm isinitialized with P_(dt)=0, thus any SN can be detected. When packetsstart to be received, T_(l), T_(u) and the list of SNs are updated.Initially, at the bootstrap, the estimated detected load and utilizationare lower. Once the detected load goes over 75% and at the same time theutilization drops below 22%, it means that the curve shown in FIG. 4 haspassed over the top and hence P_(dt) is too small and should beincreased.

The decision on the value of the detection threshold happens every timea new packet is sensed on the medium. It is more dangerous tooverestimate the detection threshold than to underestimate it. If thedetection threshold is overestimated when the total input traffic is low(see the left side of the curve of FIG. 4), this means that the inputtraffic is further decreased and hence the effective output is furtherdecreased. Similarly, when the total input traffic is high (see theright side of the curve of FIG. 4), if the detection threshold isunderestimated, the total number of detected packets is increased andagain this decreases the effective output rate. However, the potentialloss when overestimating the detection threshold is higher. The linebetween low traffic and high traffic is referred to as intensitythreshold. This threshold is here exemplarily set as the combination ofa predetermined value, e.g. 75%, of load and a predetermined value, e.g.22%, of utilization. Those two values can be monitored independently andthe situation of the load or utilization being lower or higher than thepredetermined values can then be used to trigger the adaptation of thedetection threshold. It is possible to logically combine the two valueswith an AND, OR or XOR operator and thereby achieve different resultingpacket reception. The type of combination can be selected different forthe cases of increasing versus decreasing the detection threshold.

An AND connection for increasing traffic leads to a so calledconservative adaptation. A conservative increase of P_(dt) is preferablyperformed: if it happens e.g. 4 times in a row that the detected load ishigher than 75% and the utilization lower than 22%, only then will thedetection threshold be increased. The detection threshold is increasedsuch that one SN less will be detected from thereon. In other words, ifit is assumed that P_(dt)=P^(rcv) _(i), then P_(dt)=P^(rcv) _(i−1), isupdated if i>1.

On the other hand, when decreasing the detection threshold, the processcan be less conservative. The first moment when the detected load islower then 75% and the utilization is lower than 22%, then P_(dt) is setto P_(dt)=P^(rcv) _(i+1) if i<n, or else P_(dt)=0.

The main reason for doing a conservative increase of the threshold isbecause the curve of FIG. 4 is steeper for low traffic intensity. Thedetected traffic intensity depends on the detection threshold. Thehigher the detection threshold is, the lower the detected traffic is.

If the traffic is overestimated, the detected traffic will be decreased.This will most likely happen when the total traffic is already low,hence when the operating point is on the left side of the peak. Sincethe curve is steeper, the utilization is going to suffer more strongly,and this should be avoided.

On the other hand, if the aggregate traffic is underestimated, it willbe on the right side of the curve of FIG. 4. There, the steepness of thecurve is smaller, hence an underestimate of the threshold will cause asmaller change in utilization, and it is possible to be lessconservative.

It is also advantageous to keep updating T_(l) and T_(u) even when nopacket arrivals are being detected. In the event when the trafficintensity suddenly drops, or nearby nodes cease transmitting, thedetection threshold might be too high. If a new packet arrives at timeT_(now), the following values of λ_(a), {overscore (X)}are taken:$\lambda_{a} = \frac{packet\_ duration}{{\alpha\quad T_{l}} + {\left( {1 - \alpha} \right)\left( {T_{now} - T_{end}} \right)}}$$\overset{\_}{X} = \frac{packet\_ duration}{{\alpha\quad T_{u}} + {\left( {1 - \alpha} \right)\left( {T_{now} - T_{end\_ succ}} \right)}}$

This way, the detection threshold will gradually drop in time whilethere is no detected packet.

FIG. 6 is a flow diagram of a method of receiving data packets at acluster head from transmitting devices in a sensor network. The methodincludes sensing 601 data packets transmitted on a communication medium.The total traffic intensity of data packets generated by thetransmitting devices is estimated 602. A detection threshold is adapted603 for receiving data packets as a function of the total trafficintensity.

It is determined 604 if the received signal strength of a received datapacket is greater than or equal to the detection threshold. If it isgreater than or equal to the threshold, the data packet is received 605.If it is less than the threshold, the data packet is ignored 606.

The following is the pseudo code of operations. Operationstart_transmission(j) is called at a CH when a packet from node j issensed. Operation end_transmission(j) is called when a packettransmission is finished. End_transmission(j) is called only if a packetwas detected, i.e. the received signal strength is above the detectionthreshold P_(dt). start transmission (from node j):   lambda_a =packet_duration    / (ALPHA * T_l + (1−ALPHA) * (now − T_end));   X =packet_duration    / (ALPHA * T_u + (1−ALPHA) * (now − T_end_succ));  if (X < 0.22) count = count + 1;   else count = 0;   if (lambda_a <75% and X < 22%)   begin     i = i+1;     P_dt = Prcv_i;     count = 0;  end   else if (count >= 4 and lambda_a > 75% and X < 22%)   begin    i = i−1;     P_dt = Prcv_i;     count = 0;   end   if (Prcv_j >=P_dt)   begin     // Receive     T_l = ALPHA * T_l + (1−ALPHA) * (now −T_end);     lambda_a = packet_duration / T_l;   end end_transmission:  if (successfully_received)   begin     t_u = ALPHA * t_u + (1−ALPHA) *(now − last_rcv);     util = packet_duration / utime;     T_end_succ =now;   end   if (packet_was_detected) T_end = now;

FIGS. 7A and 7B are flow diagrams showing the steps of the abovealgorithms. Referring to FIG. 7A, the transmission is started 701, andthe total traffic intensity λ_(a) and utilization {overscore (X)} areupdated 702. It is determined if the utilization is less than 22% 703;if it is, the count is equal to zero 704, if not the count isincremented by one 705.

It is then determined 706 if the total traffic intensity is less than75% and the utilization is less than 22%. If yes, the detectionthreshold is decreased 707. If not, it is determined 708 if the count isgreater than or equal to 4 and the total traffic intensity is greaterthan 75% and the utilization is less than 22%. If yes, the detectionthreshold is increased 709.

It is determined 710 if the signal strength of the packet received fromSN j is greater than or equal to the detection threshold. If yes, thesignal packet is received 711 and the average idle time of the receiverT_(l) and the total traffic intensity are updated. The transmission thenends 712.

If the signal strength is less than the detection threshold, no packetis received and the transmission ends 712.

Referring to FIG. 7B, when a transmission ends 712, it is determined 713if the packet was successfully received. If yes, the average timebetween two successful receptions T_(u), the utilization and the time ofthe end of the last successful packet transmission T_(end) _(—) _(succ)are updated 714 and the process ends 715. If the transmission is notsuccessful, the process ends 715.

As shown in FIG. 5, the main reason for a too low efficiency of a CH isthat if it starts receiving a weaker packet, and a stronger packetarrives during this reception, the weaker packet will be dropped due tothe interference, and the stronger packet will not be received becausethe receiving circuit was busy when the packet arrived. The most generalway to solve this problem is to include several receiving circuits inparallel so that a CH can cope with all arriving packets. However, thisis not necessary in most of the cases. A solution is provided thatoffers a similar performance while being simpler and cheaper toimplement.

Referring to FIG. 8 a second CH architecture is shown. A CH 104 has twodetection and synchronization circuits 801, 802. Each one of thedetection and synchronization circuits 801, 802 can detect andsynchronize with an arriving packet from a SN 102. There iscommunication between the two circuits 801, 802 in order to avoid bothcircuits 801, 802 detecting the same packet. Detection of the samepacket by both circuits 801, 802 is a risk when both circuits 801, 802are idle.

Once one of the detection and synchronization circuits 801, 802synchronizes to a packet, it forwards the data to a selectionmultiplexer 803. The selection multiplexer 803 has both signals asinputs and it selects one of the signals to forward to the receivingcircuit 804. The selection is based on a control input 805 set by acomparator 806.

The comparator 806 takes both input signals from the two detection andsynchronization circuits 801, 802, estimates the powers, and selects theone with a higher power. The comparator 806 then sets the control input805 to the selection multiplexer 803 accordingly. Also, when thecomparator 806 detects an arrival of a stronger input signal, it changesthe output of the selection multiplexer 803. The comparator 806 alsoinforms the receiving circuit 804 of the change, so that the receivingcircuit 804 knows it is switching to a new packet.

In this way, if a transmission of a new packet starts whose signal isstronger than the signal of the packet currently being received, and ifit overlaps with the current packet, the CH 104 stops receiving thecurrent packet and switches to the new, stronger packet.

FIG. 9 is a flow diagram of the method of receiving a signal packetusing the second CH architecture in the form of a switching receiver901. A packet is detected 902 and it is determined 903 if a packet hasarrived. If no packet has arrived, the receiver continues to detectpackets. Packet receiving is performed 904. It is then determined 905 ifa stronger packet is detected. If yes, the receiver switches toreceiving the stronger packet 906 and the method loops to the step ofreceiving 904. If no stronger packet is detected, it is determined 907if the reception is finished. If it is finished, the method loops todetecting further packets 902. If it is not finished, the method loopsto receiving 904 the packet.

The approach with the adaptive detection threshold yields an improvementcomparing to known architectures, while maintaining the same level ofarchitectural complexity. The switched architecture introduces anadditional performance improvement but with a slight increase in CHproduction cost.

It is also possible to use a method for receiving data from a pluralityof transmitting devices at a receiving device, comprising: detecting afirst data packet with a first signal strength; detecting a second datapacket with a second signal strength; comparing the first and secondsignal strengths; receiving the data packet with the stronger signalstrength. The first and second data packets may be detected in parallel.The method may include: receiving a first data packet with a firstsignal strength; detecting a second data packet with a second signalstrength during the receiving process of the first data packet;determining if the second signal strength is greater than the firstsignal strength; discontinuing the receipt of the first data packet andswitching to the second data packet if the second signal strength isgreater than the first signal strength.

This method can be performed by a receiving device for receiving datapackets from a plurality of transmitting devices, comprising: first andsecond detection circuits for detecting the arrival of data packets; acontroller for comparing the signal strengths of detected data packetsand selecting a data packet with the strongest signal strength; areceiving circuit for receiving the selected data packet. The detectioncircuit may detect the arrival of a second data packet with a greatersignal strength than the signal strength of a first data packet in theprocess of being received by the receiving circuit, and the controllerinstructs the receiving circuit to discontinue the receipt of the firstdata packet and switch to the second data packet. The controller mayinclude a comparator for estimating and comparing the signal strengthsof data packets and a selection multiplexer for forwarding the selecteddata packet to the receiving circuit.

Any of the above embodiments can be combined in part or as a whole. Thepresent invention can be implemented as a computer program product,comprising a set of program instructions for controlling a computer orsimilar device. These instructions can be supplied preloaded into asystem or recorded on a storage medium such as a CD-ROM, or madeavailable for downloading over a network such as the Internet or amobile telephone network.

Improvements and modifications can be made to the foregoing withoutdeparting from the scope of the present invention. Variations describedfor the present invention can be realized in any combination desirablefor each particular application. Thus particular limitations, and/orembodiment enhancements described herein, which may have particularadvantages to a particular application need not be used for allapplications. Also, not all limitations need be implemented in methods,systems and/or apparatus including one or more concepts of the presentinvention. Methods may be implemented as signal methods employingsignals to implement one or more steps. Signals include those emanatingfrom the Internet, etc.

The present invention can be realized in hardware, software, or acombination of hardware and software. A visualization tool according tothe present invention can be realized in a centralized fashion in onecomputer system or in a distributed fashion where different elements arespread across several interconnected computer systems. Any kind ofcomputer system—or other apparatus adapted for carrying out the methodsand/or functions described herein—is suitable. A typical combination ofhardware and software could be a general purpose computer system with acomputer program that, when being loaded and executed, controls thecomputer system such that it carries out the methods described herein.The present invention can also be embedded in a computer programproduct, which comprises all the features enabling the implementation ofthe methods described herein, and which—when loaded in a computersystem—is able to carry out these methods.

Computer program means or computer program in the present contextinclude any expression, in any language, code or notation, of a set ofinstructions intended to cause a system having an information processingcapability to perform a particular function either directly or afterconversion to another language, code or notation, and/or reproduction ina different material form.

Thus the invention includes an article of manufacture which comprises acomputer usable medium having computer readable program code meansembodied therein for causing a function described above. The computerreadable program code means in the article of manufacture comprisescomputer readable program code means for causing a computer to effectthe steps of a method of this invention. Similarly, the presentinvention may be implemented as a computer program product comprising acomputer usable medium having computer readable program code meansembodied therein for causing a function described above. The computerreadable program code means in the computer program product comprisingcomputer readable program code means for causing a computer to affectone or more functions of this invention. Furthermore, the presentinvention may be implemented as a program storage device readable bymachine, tangibly embodying a program of instructions executable by themachine to perform method steps for causing one or more functions ofthis invention.

It is noted that the foregoing has outlined some of the more pertinentobjects and embodiments of the present invention. This invention may beused for many applications. Thus, although the description is made forparticular arrangements and methods, the intent and concept of theinvention is suitable and applicable to other arrangements andapplications. It will be clear to those skilled in the art thatmodifications to the disclosed embodiments can be effected withoutdeparting from the spirit and scope of the invention. The describedembodiments ought to be construed to be merely illustrative of some ofthe more prominent features and applications of the invention. Otherbeneficial results can be realized by applying the disclosed inventionin a different manner or modifying the invention in ways known to thosefamiliar with the art.

1. A method for receiving data packets from a plurality of transmitting devices at a receiving device, comprising: sensing data packets transmitted on a communication medium; estimating the total traffic intensity of data packets generated by the transmitting devices; adapting a detection threshold for receiving data packets as a function of the total traffic intensity; ignoring data packets with a received signal strength below the current detection threshold; and receiving data packets with a received signal strength above the current detection threshold.
 2. A method as claimed in claim 1, wherein when the total traffic intensity is low, the detection threshold is decreased.
 3. A method as claimed in claim 1, wherein when the total traffic intensity increases, the detection threshold is increased.
 4. A method as claimed in claim 1, wherein the detection threshold is adapted for every data packet sensed.
 5. A method as claimed in claim 1, wherein the method includes tracking and estimating the traffic intensity and the utilisation at the receiving device and adapting the detection threshold to keep the utilisation at a maximum.
 6. A method as claimed in claim 5, wherein the traffic intensity is estimated by a packet duration divided by the average idle time at the receiving device.
 7. A method as claimed in claim 5, wherein the utilisation at the receiving device is estimated by a packet duration divided by the average time between two successful packet receptions.
 8. A method as claimed in claim 5, wherein the estimated traffic intensity and utilisation are continually updated.
 9. A method as claimed in claim 5, wherein the method includes maintaining a list of active transmitting devices and their received signal strengths.
 10. A method as claimed in claim 1, wherein the total traffic intensity is maintained at approximately 75% of the physical layer data rate and the utilization is maintained at approximately 22%.
 11. A method as claimed in claim 1, wherein the detection threshold is conservatively increased.
 12. A method as claimed in claim 11, wherein the detection threshold is increased if it is detected a predetermined number of times in succession that the total traffic intensity is greater than 75% and the utilization is less than 22%, and wherein the detection threshold is decreased if the total traffic intensity is less than 75% and the utilization is less than 22%.
 13. An article of manufacture comprising a computer usable medium having computer readable program code means embodied therein for causing reception of data packets from a plurality of transmitting devices at a receiving device, the computer readable program code means in said article of manufacture comprising computer readable program code means for causing a computer to effect the steps of claim
 1. 14. A receiving device for receiving data packets from a plurality of transmitting devices, comprising: a receiving circuit for receiving data packets with a received signal strength above a current detection threshold; estimator for the total traffic intensity of data packets generated by transmitting devices; controller for adapting the detection threshold for receiving data packets as a function of the total traffic intensity.
 15. A device as claimed in claim 14, including at least one of: a sensor for data packets on a communication medium at the receiving device, and a list of active transmitting devices and their received signal strengths.
 16. A computer program product comprising a computer usable medium having computer readable program code means embodied therein for causing reception of data packets from a plurality of transmitting devices at a receiving device, the computer readable program code means in said computer program product comprising computer readable program code means for causing a computer to effect the functions of claim
 14. 17. A computer program product stored on a computer readable storage medium, comprising computer readable program code means for performing the steps of: sensing data packets transmitted on a communication medium; estimating the total traffic intensity of data packets generated by the transmitting devices; adapting a detection threshold for receiving data packets as a function of the total intensity; ignoring data packets with a received signal strength below the current detection threshold; and receiving data packets with a received signal strength above the current detection threshold.
 18. A sensor network comprising: a plurality of wireless sensors having transmitters; one or more receiving devices; and a central server for receiving data from the one or more receiving devices; wherein the one or more receiving devices comprising: a receiving circuit for receiving data packets from the sensors, the data packets having a signal strength above a current detection threshold; an estimator for the total traffic intensity of data packets generated by the sensors; a controller for adapting the detection threshold for receiving data packets as a function of the total traffic intensity; and a transmitter for transmitting received data to the central server.
 19. A sensor network as claimed in claim
 18. wherein the network operates on a pulse-based ultra-wide band physical layer.
 20. A method for receiving data from a plurality of transmitting devices at a receiving device, comprising: detecting a first data packet with a first signal strength; detecting a second data packet with a second signal strength; comparing the first and second signal strengths; receiving the data packet with the stronger signal strength.
 21. A method as claimed in claim 20, wherein the first and second data packets are detected in parallel.
 22. A method as claimed in claim 20, wherein the method includes receiving a first data packet with a first signal strength; detecting a second data packet with a second signal strength during the receiving process of the first data packet; determining if the second signal strength is greater than the first signal strength; discontinuing the receipt of the first data packet and switching to the second data packet if the second signal strength is greater than the first signal strength.
 23. A receiving device for receiving data packets from a plurality of transmitting devices, comprising: first and second detection circuits for detecting the arrival of data packets; a controller for comparing the signal strengths of detected data packets and selecting a data packet with the strongest signal strength; a receiving circuit for receiving the selected data packet.
 24. A receiving device as claimed in claim 23, wherein a detection circuit detects the arrival of a second data packet with a greater signal strength than the signal strength of a first data packet in the process of being received by the receiving circuit, and the controller instructs the receiving circuit to discontinue the receipt of the first data packet and switch to the second data packet.
 25. A receiving device as claimed in claim 23, wherein the controller includes a comparator for estimating and comparing the signal strengths of data packets and a selection multiplexer for forwarding the selected data packet to the receiving circuit. 